Imagine a rotating sphere that is 12,800 kilometers (8000 miles) in
diameter, has a bumpy surface, is surrounded by a 40-kilometer-deep
mixture of different gases whose concentrations vary both spatially and
over time, and is heated, along with its surrounding gases, by a nuclear
reactor 150 million kilometers (93 million miles) away. Imagine also
that this sphere is revolving around the nuclear reactor and that some
locations are heated more during one part of the revolution and other locations are heated
during another part of the revolution. And imagine that this mixture of
gases continually receives inputs from the surface below, generally
calmly but sometimes through violent and highly localized injections.
Then, imagine that after watching the gaseous mixture, you are expected
to predict its state at one location on the sphere one, two, or more
days into the future. This is essentially the task encountered day by
day by a weather forecaster (Ryan, Bulletin of the American
Meteorological Society,1982).

Earth rotates on its axis once every 23 hours, 56 minutes, and completes one revolution around the sun every 365.25 days. (Courtesy of Rob Simmon.)